My invention relates to a time-division multiplex, pulse-code modulation system, and particularly to an arrangement that permits all of the sequential pulses of one or more relatively wide band (or voice) channels to be used to transmit information, particularly for data channels.
Communication systems using time-division multiplexing and pulse-code modulation are used to provide a plurality of relatively low noise, easily regenerated communication channels over a single communication circuit. One such system, designated the D2/D3 system by the Bell Telephone System, is described in considerable detail in "The Bell System Technical Journal," Volume 51, October 1972, beginning at page 1641. The D2/D3 system provides 24 relatively wide band (approximately 180 to 3,400 hertz) voice channels over two T1 lines, one line being used for each direction of transmission. In that system, the polarity and amplitude of the information in each of the 24 channels is sampled 8,000 times per second. This polarity and amplitude sample is encoded into eight binary pulses or bits, one bit representing polarity and the other seven bits representing amplitude levels from 64 down to one. The bits are sequentially combined for each channel, and the combined sequences for each of the 24 channels are sequentially combined to form what is designated a frame. In some applications, it is desirable or necessary that one or more of the relatively wide band voice channels be dedicated to or used for transmission of either wide or narrow band information or data. The pulses of each such dedicated channel can be used to provide a single wide band data channel or a plurality of narrower band data channels. The data rate of each data channel determines the needed band width or number of pulses for that channel. At first glance, it would appear that all eight pulses in the dedicated voice channel can be used for transmitting data. However, repeaters between a transmitter and receiver require that a pulse of selected characteristic or polarity be transmitted at least once during a sequence of eight pulses. Accordingly, previous systems which dedicated one or more wide band voice channels to information or data channels required that if N bits or pulses were present in a given channel, then only (N - 1) bits or pulses could be used for transmitting data or information, and that the other or Nth bit or pulse had to be reserved to transmit the necessary keep-alive bit or pulse for the repeaters in the system.
Accordingly, a primary object of my invention is to provide a new arrangement for use in a multi-channel, timedivision multiplex, pulse-code modulation system that permits all of the pulses or bits in a selected channel to be used for transmitting data or information.
Another object of my invention is to provide a new arrangement which permits all N pulses of a sequence of pulses to be used for transmitting data or information, and which provides the necessary keep-alive pulse when all but one pulse in a sequence fail to provide the necessary or keep-alive pulse.
Another and relatively specific object of my invention is to provide a novel transmitting circuit which detects the absence of a keep-alive pulse in all but one pulse in each sequence and provides such a keep-alive pulse in the time slot of that one pulse, and which also detects the presence of a keep-alive pulse in each sequence and permits all pulses in the sequence to represent information.
Another and relatively specific object of my invention is to provide a novel receiving circuit which detects the absence of a keep-alive pulse in all but one pulse in each sequence and causes a repetition of the previous pulse in the time slot of that one pulse, and which detects the presence of a keep-alive pulse in each sequence and permits all pulses in that sequence to be utilized.